In the field of optical microscopy, in order to obtain a meaningful image, the microscope must be focused on the object, or specimen, being examined. During the time that the microscope is focusing on the specimen, use of the microscope is unavailable. For this reason, numerous efforts have been made to minimize focusing time, as well as to automate the process. This is generally referred to as auto-focusing (AF).
Since the introduction of the automated optical inspection (AOI) of media, AF has become more prevalent in the industry. AOI instruments are broadly used and, in particular, have become instrumental in the inspection of flat panel displays (FPD) during the manufacturing process and the inspection of silicon wafers during the production of integrated circuits. In both of these cases, the specimen being inspected is very large in comparison to the field of view of the microscope which is typically 0.5 mm by 0.5 mm.
In order for the microscope to examine specimens which are larger than its field of view, two techniques have generally been employed. A first technique is performed by focusing the microscope on previously identified spots which are suspected to be defective while the second technique is to systematically scan the entire specimen. Efficiently of both of these AOI techniques is severely restricted by the performance of presently available AF equipment.
It is, therefore, desirable to provide a novel method and apparatus for auto-focusing infinity corrected microscopes.